Cannot read into the underlying string

ext/java/org/jruby/ext/stringio/StringIO.java

@@ -1110,7 +1110,7 @@ private IRubyObject readCommon(ThreadContext context, int argc, IRubyObject arg0
             final RubyString string;
             switch (argc) {
                 case 2:
-                    str = readBuf(arg1);
+                    str = readBuf(context, ptr, arg1);
                 case 1:
                     if (!arg0.isNil()) {
                         len = RubyNumeric.fix2int(arg0);
@@ -1202,7 +1202,7 @@ private RubyString preadCommon(ThreadContext context, int argc, IRubyObject arg0
 
         switch (argc) {
             case 3:
-                str = readBuf(arg2);
+                str = readBuf(context, ptr, arg2);
             case 2:
                 len = RubyNumeric.fix2int(arg0);
                 if (!arg0.isNil()) {
@@ -1694,12 +1694,16 @@ private static IRubyObject substrString(RubyString ch, IRubyObject str, Ruby run
         return str;
     }
 
-    private static IRubyObject readBuf(IRubyObject str) {
-        if (!str.isNil()) {
-            str = str.convertToString();
-            modifyString((RubyString) str);
+    private static IRubyObject readBuf(ThreadContext context, StringIOData ptr, IRubyObject arg) {
+        if (!arg.isNil()) {
+            arg = arg.convertToString();
+            RubyString str = (RubyString) arg;
+            modifyString(str);
+            if (str == ptr.string) {
+                throw context.runtime.newArgumentError("cannot read into the underlying string");
+            }
         }
-        return str;
+        return arg;
     }
 
     // MRI: strio_write

ext/stringio/stringio.c

@@ -167,6 +167,9 @@ strio_readbuf(struct StringIO *ptr, VALUE str)
     if (!NIL_P(str)) {
 	StringValue(str);
 	rb_str_modify(str);
+	if (str == ptr->string) {
+	    rb_raise(rb_eArgError, "cannot read into the underlying string");
+	}
     }
     return str;
 }

test/stringio/test_stringio.rb

@@ -765,6 +765,8 @@ def test_read
     s = ""
     f.read(nil, s)
     assert_equal(Encoding::ASCII_8BIT, s.encoding, bug20418)
+
+    assert_raise(ArgumentError) {f.read(1, f.string)}
   end
 
   def test_readpartial
@@ -830,6 +832,7 @@ def test_pread
 
     assert_raise(EOFError) { f.pread(1, 5) }
     assert_raise(ArgumentError) { f.pread(-1, 0) }
+    assert_raise(ArgumentError) { f.pread(0, 0, f.string) }
     assert_raise(Errno::EINVAL) { f.pread(3, -1) }
     assert_raise(Errno::EINVAL) { f.pread(0, -1) }
     assert_raise(IOError) { StringIO.new(nil, "w").pread(3, 0) }